Zinc alpha-2-glycoprotein (ZAG) is a plasma glycoprotein that was named for its electrophoretic mobility and for its ability to be precipitated by Zn salts (Burgi and Schmid 1961). It is a member of the immunoglobulin gene superfamily and has a three-dimensional structure that is highly homologous to MHC class I and II molecules (Sanchez et al 1999). ZAG has been detected immunohistochemically in normal secretory epithelial cells of breast, prostate, and liver, in salivary, bronchial, gastrointestinal, and sweat glands (Tada et al 1991; Hale et al 2001), and in normal stratified epithelia including epidermis (Lei et al 1997; Brysk et al 1997b). ZAG mRNA is expressed in a similar distribution (Freije et al 1991). Consistent with its production by secretory epithelium, ZAG is present in most body secretions and constitutes 2.5% and 30% of the proteins present in saliva and seminal fluid respectively (Poortsmans and Schmid 1968). Plasma or serum levels of ZAG have been described to vary with age, with reported values ranging from 0.9-3.5 mg/dl (fetal) to 7.8-12.1 mg/dl (young adults) to 18-30 mg/dl (normal men aged 51-70)(Jirka et al 1974; Jirka et al 1978; Hale et al 2001). ZAG accumulates in breast cyst fluids to 30-50-fold plasma concentration (Bundred et al 1987; Sanchez et al 1997) and is over-expressed in 40-50% of breast carcinomas (Bundred et al 1987; Sanchez et al 1992; Diez-Itza et al 1993). It has recently been shown that ZAG is produced in high amounts by most prostate carcinomas, resulting in elevated serum levels of ZAG in prostate cancer patients (Hale et al 2001). ZAG has also been identified in epidermal malignancies, including squamous and Merkel cell carcinomas, with lesser expression in basal cell carcinomas (Brysk et al 1997b; Lei et al 1997). Little is known about mechanisms regulating ZAG expression by tumors, however ZAG production by normal epithelial tissues was shown to be increased by treatment with androgens, corticosteroids, interferon-α, or TGF-α (Lopez-Boado et al 1994; Brysk et al 1997; Brysk et al 1997b).
The normal functions of ZAG are unclear, however ZAG has been isolated from the urine of human cancer patients with cachexia and can function as a lipid-mobilizing factor. ZAG purified from human or mouse serum or human cancer patient urine induces lipolysis resulting in glycerol release and also increases lipid utilization in both human and mouse adipocytes (Hirai et al 1998). ZAG activates a GTP-dependent adenylate cyclase activity on adipocyte membranes, increasing cellular cAMP levels (Hirai et al 1998). This may potentially lead to activation of multiple cellular pathways, however further details of its mechanism of action are unknown. Using a panel of murine tumors, Todorov et al (1998) quantitated ZAG production in vitro and showed that cachexia induction in mice bearing these tumors in vivo correlated with their ZAG production. Whether ZAG has additional biological activities in addition to cachexia induction is unknown.
To further investigate biologic properties of ZAG, stable transfectants of recombinant human (rh)ZAG were created in the B16F10 murine melanoma cell line. The effect of ZAG transfection on melanin production was determined in vitro and in vivo. The effect of exogenous ZAG on melanin production by parent B16 cells and melan-A primary melanocytes was determined. Finally, the effect of ZAG on tyrosinase expression and activity was determined. Taken together, these studies show that ZAG inhibits melanin production in both normal and malignant melanocytes. Mechanisms include post-transcriptional effects on tyrosinase protein, with the potential for additional indirect effects. These studies resulted in the identification of a previously unknown biologic function of ZAG and have made possible a method of modulating melanin production and thereby preventing and/or decreasing pigmentation of skin and hair due to increased melanin production.